The control circuits of stepping motors may take numerous different forms. Their function however is always that of applying energization pulses to the winding, i.e. pulses during which the winding is coupled to an energization source.
These energization pulses may have durations more or less long. Among them, for instance, one may distinguish motor drive pulses which are those normally intended to cause the rotor to step, catch-up pulses having a duration generally greater than that of the drive pulses, and check pulses which are generally of a duration insufficient to cause the rotor to step.
Such energization pulses cause a current to flow in the winding of which the variation as a function of time is well known and will not be described here.
In most cases the control circuit of the motor short-circuits the winding between energization pulses and in particular following each motor drive pulse or catch-up pulse. Because of this short-circuit, a current likewise circulates in the winding immediately following the end of these pulses. This current is induced in the winding by the oscillations which the rotor effects about its equilibrium position at the end of its step. This current is thus likewise produced indirectly by the energization pulse which causes the rotor to step.
The control circuits of stepping motors frequently comprise means for providing a measurement signal representing the current which circulates in the motor winding responsive to these energization pulses. This measurement signal may be employed for instance to adjust the quantity of energy furnished to the motor by its control circuit as a function of the mechanical load driven by the motor. This adjustment enables diminishing the overall energy consumed by the motor, this being important when the motor forms part of a portable arrangement such as an electronic time-piece in which the space available for the energy source is very limited. U.S. Pat. No. 4,114,364 describes a circuit bringing about such adjustment for each motor drive pulse, by measurement of the current which circulates in the winding responsive to the motor drive pulse. In such case, the measurement of the current must be made over the entire duration of the motor drive pulse.
U.S. Pat. No. 4,212,156 describes another circuit realizing this adjustment based on measurement of the current circulating in the winding when the latter is short-circuited, immediately following a motor drive pulse. In this case the current measurement must thus be effected at the end of the motor drive pulse.
The current measurement in the winding may likewise serve to determine if the rotor has stepped correctly responsive to a motor drive pulse. U.S. Pat. No. 4,272,837 describes a control circuit which, following each motor drive pulse, applies to the winding a check pulse having a duration insufficient to step the rotor. The rate of growth of the current in the winding responsive to such check pulse permits the control circuit to determine whether the rotor has correctly stepped or not. In the latter case, the control circuit provides a supplemental pulse, generally referred to as a catch-up pulse, such that the rotor carries out the step which it has just missed. The control circuit likewise adjusts the energy provided to the motor by the following motor drive pulses. In this case, the current measurement must not be made except during the check pulse.
This determination of the rotation or non-rotation of the rotor responsive to a motor drive pulse may likewise be obtained by measuring the current which circulates in the winding responsive to the following motor drive pulse, at an instant situated between the beginning of such motor drive pulse and the beginning of the stepping of the rotor. U.S. Pat. No. 4,300,223 describes a circuit functioning in such manner. In this case, the current measurement must be made only at the instant mentioned hereinabove.
The means for measuring current circulating in the winding generally comprise a resistance coupled in series with the winding. The voltage drop produced by the passage of the current in this resistance constitutes the measurement signal of the current.
Such resistance brings about a lowering of the voltage at the motor terminals and dissipates an energy which is not negligible. To avoid these difficulties, the resistance value must be chosen to be very small but in such case the voltage drop at its terminals is likewise very small thus causing difficulties for its utilization by the measurement circuit.
It has also been proposed to employ the voltage drop at the terminals of one of the power transistors coupled to the winding as a measurement of the current circulating in the latter. The suppression of the measurement resistance effectively comprises an advantage. But the voltage drop at the terminals of the transistor is small, this likewise causing difficulties for its utilization for the control circuit. Moreover, this voltage drop will vary from one transistor to another because of the inevitable differences in their characteristics, this also bringing about difficulties for its utilization.
The known means thus do not permit obtaining measurement of the current circulating in the motor winding in a simple and precise manner.
The purpose of the present invention is to propose a control circuit for a stepping motor comprising a measurement circuit for the current circulating in the winding which avoids the difficulties mentioned hereinabove.